9 lg. egg yolks
1 c. sugar
6 tbsp. cornstarch
3 c. milk
1 tbsp. vanilla
14 oz. fine quality bittersweet chocolate, chopped
4 c. well chilled heavy cream
14 oz. fine quality white chocolate, chopped

FOR GARNISH:

6 oz. fine quality white chocolate, room temperature (about 72 degrees)
4 oz. fine quality bittersweet chocolate, room temperature (about 72 degrees)

In a large bowl whisk together the egg yolks and the sugar until the mixture is combined well, add the cornstarch, sifted, and whisk the mixture until it is just combined. Whisk in the milk, scalded, in a slow stream, transfer the mixture to a heavy saucepan, and bring it to a boil, whisking constantly. Boil the pastry cream, whisking, for 1-2 minutes or until it is very thick and smooth, strain it through a fine sieve into a heatproof bowl, and stir in the vanilla. Lay a piece of plastic wrap on the pastry cream and chill the pastry cream for 1-2 hours, or until it is cooled completely. I usually transfer it to a shallow pan or dish, it cools quicker and will thicken more as it cools.

In the top of a double boiler or a metal bowl set over barely simmering water melt the bittersweet chocolate, stirring occasionally, and let it cool until it is lukewarm. Transfer half the pastry cream to a large bowl, whisk in the chocolate, and whisk the mixture until it is combined well.

In a chilled bowl with an electric mixer beat 2 cups of the heavy cream until it holds soft peaks, stir (use whisk, helps lighten and aerate) 1/4 of it into the chocolate mixture and fold in (use spatula, won't deflate) the remaining whipped cream gently but thoroughly. Pour the dark chocolate mousse into a lightly oiled 9 or 10 inch springform pan, at least 2 1/2 inches deep, spread it evenly, and chill it while making the white chocolate mousse. 9 inch works better, makes it taller.

In the top of a double boiler or a metal bowl set over barely simmering water melt the white chocolate, stirring occasionally, let it cool for 2-3 minutes, or until it is lukewarm, and whisk it into the remaining pastry cream until the mixture is combined well.

In a chilled bowl with an electric mixer beat the remaining 2 cups heavy cream until it holds soft peaks, stir 1/4 of it into the white chocolate mixture, and fold in the remaining whipped cream gently but thoroughly. Pour the white chocolate mousse over the chilled dark chocolate mousse, spread it evenly, and chill the cake, covered loosely with parchment or wax paper, or overnight.

Prepare the garnish: Grate fine 2 ounces of the white chocolate and reserve it. Working over a foil-lined baking sheet or tray, shave curls from the remaining 4 ounces white chocolate and from the bittersweet chocolate with a vegetable peeler, keeping the white and dark curls separate, and chill them, covered loosely, for at least 1 hour or overnight.

Run a thin knife around the edge of the cake, remove the side of the pan, and smooth the side of the cake with a spatula. Put the cake on a rack set in a shallow baking pan, press the grated white chocolate onto the side, gathering the excess in the pan and reapplying it until the side is coated evenly and transfer the cake to a platter.